The field of the disclosure relates generally to fuel systems and, more particularly, to methods and systems for enhancing fuel tank safety.
Some known fuel tanks have a liquid fuel containing region that typically holds fuel and an ullage region that typically contains a mixture of air and evaporated fuel (i.e. fuel vapor) that defines an fuel/air ratio within the ullage. Ignition of the fuel/air mixture within the ullage may occur, in the presence of an ignition source, when the concentration of fuel vapors (i.e. fuel/air ratio) in the ullage lies within a certain range, commonly known as a combustion supporting range, an unsafe region or a non-inert region. More specifically, the lower flammability limit of the ullage is defined as a threshold below which the fuel/air ratio is too lean and will not ignite. Additionally, the upper flammability limit of the ullage is defined as the threshold above which the fuel vapor/air mixture is too rich to ignite. The lower limit represents the minimum fuel vapor/air mixture concentration that will ignite, while the upper limit represents the maximum fuel vapor/air mixture concentration that will support combustion. A combustion supporting region for a fuel/air mixture is defined between the lower limit concentration and the upper limit concentration. The mixture generally is not combustible outside of this region.
Under typical operating conditions, for example temperatures less than 100° F. at sea level, the fuel/air mixture concentration for Jet A fuel lies outside of the non-inert region and therefore is generally not combustible. However, there are a number of known events that may cause the inert fuel/air mixture within the ullage region of the fuel tank to enter the unsafe region. These circumstances may include, for example, a rapid reduction in tank ullage pressure after take off, i.e. when the aircraft reaches a high altitude in a short time when the fuel is still at the temperature that existed at take-off (for example, 98° F.). This may cause the ullage fuel vapor/air mixture concentration to enter the unsafe region at the higher altitude.
Recent Federal Aviation Administration (FAA) Regulations require that new transport aircraft include systems for enhancing the safety of aircraft fuel tanks. One known system for increasing the reliability of aircraft fuel tanks is to utilize an “inerting system” that channels an inert gas, such as nitrogen or nitrogen enriched air (NEA), into the fuel tank to reduce the oxygen concentration therein. NEA may be generated on-board using, for example, high pressure bleed air from an engine compressor or an auxiliary power unit compressor. In either case, the high pressure air flows through equipment that removes contaminants and moisture, and conditions the air to pressures and temperatures required by the air separation modules that separate the air into an oxygen-rich component that is exhausted from the aircraft and an oxygen-depleted or nitrogen enriched component that flows into the fuel tank. Such a system is expensive to install on an aircraft, significantly increases the weight of the vehicle, and also may not be reliable due to its complexity. Inerting systems, in general, vent fuel vapor-laden ullage gases to the outside ambient when supplying oxygen-depleted or inert gas to the fuel tank. Additionally, aircraft descent rate may impact inerting system design, wherein a high descent rate may impact inert gas flow in order to limit or prevent outside air from entering the fuel tank and maintain the inert state of the fuel tank. This may require large quantities of bleed air to be channeled to the on-board inert gas generating system.
Another known system for enhancing the safety of a fuel tank is to maintain the fuel tank at a relatively low temperature that facilitates preventing fuel vaporization and hence formation of fuel vapors in the fuel tank. One known method for doing so involves using an air conditioning system to displace warm air surrounding the fuel tank.